Grad Publication: Yasmeen Hussain and Making Spray-and-Pray Pay

Have you ever wondered how sea urchins reproduce? Do you envision their spiny selves crawling over to other spiny friends, courting by poking each other, mysterious appendages emerging from their cryptic bodies?

Lytechinus pictus, one sea urchin species I work with
Unfortunately for your imagination, sea urchins don’t look for mates. Instead, they broadcast spawn, releasing their sperm and eggs directly into the ocean environment. Sea urchins are not alone in broadcast spawning: jellyfish, sea cucumbers, most corals, sea stars, anemones, sponges, oysters, and many other marine organisms reproduce with this “spray and pray” strategy. Sea urchin sperm are small (~25 µm long) and sea urchin eggs are slightly less small (~100 µm in diameter, about the width of a human hair). A tide pool with a volume of 1 cubic meter is gigantic in comparison to these sexy little cells – if the sperm cell was now the size of an average-height person, the length of the tidepool would be nearly the length of Rhode Island. So we might wonder: how in the world do sea urchin sperm and eggs find each other?

One way that sea urchin eggs help sperm find them in the mystifyingly large, open, and dynamic ocean is by sending out chemical signals. Sea urchin eggs release species-specific attractant chemicals (chemoattractants) that diffuse through the water and help sperm sniff out eggs. Researchers have actually been studying how sperm use chemicals (chemo-) to orient towards eggs (-taxis,) a phenomenon called chemotaxis, for years. But what wasn’t known is whether sperm from different males respond to these chemicals differently, or whether stronger sperm responses to chemoattractants really help males fertilize more eggs.

To investigate this problem, my coauthors and I conducted two studies. In the first study, we removed chemoattractants from some live eggs and compared how often these eggs were fertilized compared to eggs that still had chemoattractant. Eggs without chemoattractant were less likely to be fertilized, suggesting that sperm sniffing helps them find and fertilize eggs. Great! On to the even more exciting stuff.

Here’s the microfluidic device we use in chemotaxis tests 
In the second study, my collaborators designed a microfluidic device for chemotaxis tests. It’s “microfluidic” because it’s small (micro-) and allows us to flow liquid (-fluidic) through a channel.The device is designed to expose sperm to really specific, well-defined concentrations of chemoattractant. We put the device on a microscope and video-record sperm behavior so that we can see what direction the sperm swim. In our chemotaxis tests, sperm went in the middle channel and chemoattractant and clean seawater went on either side of the sperm. Using the recordings of sperm behavior combined with fertilization tests, we found that male urchins that have sperm that are more attracted to the chemoattractant side of the channel also fertilize more eggs. This tells us that urchin males that have sperm that are good at sniffing out eggs are also more likely to make babies. This is the first evidence that sperm chemotaxis could change the reproductive success of individual males! I hope you’re as excited about this new discovery as I am.

So, how do sea urchin sperm find and fertilize eggs in the enormous and energetic ocean environment? In this paper, we’ve shown that (in a lab) sea urchin eggs releasing attractant chemicals seems to really help sperm find eggs and fertilize them, and this likely also applies to urchins living outside the lab.

Is sperm chemoattraction the same thing as pheromones?

  • Not quite – pheromones and sperm chemoattractants are similar in that they’re both types of chemical communication, but they differ in that pheromones attract entire individuals from the same species instead of single cells.

Is this relevant to human infertility? Or contraception? Or humans at all? Who cares?

  • Maybe! Sea urchin sperm are actually not a bad proxy for human sperm. They’re about the same size, and they have similar chemoattractant receptor proteins and internal responses to sensing chemoattractants (and they don’t have the wild hooks on their heads that mouse sperm do). Now that our study shows a link between sperm chemotaxis and reproductive success, we could use our microfluidic chemotaxis tests with human sperm to better understand unexplained male infertility. Or, researchers could work to develop new contraceptives. The possibilities are endless! This is basic research – exploration for its own sake, to better understand the world we live in – which can lead us in directions we never thought possible (more on that here).

Check out the full paper here and please excuse me as I head back to work on my next project (as explained by SciPos administrator Emily Grason): “How much perfume do urchin eggs need to wear so sperm can find them in the ocean?” 


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